Some question about GIC v1 setting

[loody]

hi Russel:

2013/11/16 Russell King - ARM Linux <linux at arm.linux.org.uk>:
> On Sat, Nov 16, 2013 at 12:55:11AM +0800, loody wrote:
>> hi all:
>> I use arm cortex A9 smp with GIC v1 system.
>> And my kernel is 3.8.13.
>>
>> I have some questions:
>> 1.  From GIc v1 spec, we can modify irq priority.
>>      (ICDIPRn, Interrupt Priority Rigisters)
>>      how could we do that in kernel? Did kernel provide any API of Irq
>> to reach that goal?
>
> No.  It's pointless.  The kernel doesn't support interrupting one IRQ
> while another is already in progress.
in so far kernel, there are impossible
1. dynamically  change irq priority setting in GIC through kernel API
2. get higher interrupt priority when first time register requtest_irq

>
>> 2. From Gic V1 spec, we can assign 1 irq to multi-core.
>>      (ICDIPTRn, Interrupt Processor Targets Registers)
>>      Is there similar API in kernel to let 1 irq possibility to be
>> handled by multi-cores?
>
> No.  All that does is lead to cores being woken up and racing on locks,
> and then causing IRQs to be spuriously marked as false (because there's
> nothing for the handlers to do on the cores which lost out).
In Gic spec, if 1 core service the irq, other cores will read invalid
#1023 if accidentally they are waken up by GIC.

Why that will let cores racing on locks?

if accidentally waken up by GIC, and cores read #1023, cores can
return back immediately.

>
> If you want to redirect IRQs to different cores, run the userspace
> irqbalance daemon.


Appreciate your help,